How to get list of bm.verts that belong to a deform layer?
OK I managed to find the values I needed by printing these variables:

# Layer Deform is the vertex group layer in bmesh mode.
layer_deform = bm.verts.layers.deform.active
if layer_deform is not None:
print('--- This is the deform layer info: ---')
    for vert in bm.verts:
        dvert = vert[layer_deform]
        print('vert index: ', vert.index, 'vert id:', vert,'vertex groups:', dvert.items()) #dvert.items gives you two elements: int of vertex group (I think by the order they were created added), and a 0.0 to 1.0 about the weight. If the vert doesn't belong to any vertex group, this dict will be emtpy. vert gives the BMVert in this format: BMVert(0x0000000012998B88)
print('--- End of deform layer info ---')